1. Field of the Invention
The present invention generally relates to conversion of chemical energy to electrical energy. More particularly, this invention relates to an application and design of a defibrillator battery and, more specifically, a high capacity lithium battery designed for high rate discharge applications.
2. Prior Art
Implantable ventricular cardiac defibrillators use lithium/silver vanadium oxide (Li/SVO) electrochemical cells as their power sources. For an implantable medical device, it is preferable that the device be as small as possible, responsive to the patient's medical needs, contribute to long device service life, and the like. Therefore, when batteries are built for these medical applications, special cell designs are needed to meet their requirements. For implantable cardiac defibrillator applications, one of the most important requirements is that the power source provide high energy (25 to 40 joules) within as short a period of time as possible, and preferably within 7 to 15 seconds, or less.
The relationship between delivered energy and the cell characteristics of voltage and current is:ΔE=V×I×twhere ΔE is the delivered energy (joules), V is the cell voltage under high current pulse discharge (Volt), I is the amplitude of the pulsing current (Ampere), and t is the high current pulsing time (second). In order to provide the desired energy (ΔE) within a short time (t), the value of (V×I) needs to be kept as large as possible. Therefore, for an electrochemical cell designed to power an implantable cardiac defibrillator, the cell's internal resistance needs to be kept as low as possible, and at the same time the cell's voltage under high current pulsing needs to be maintained as high as possible.
In a traditional defibrillator cell, only silver vanadium oxide is used as the cathode active material. Silver vanadium oxide not only delivers sufficient energy needed under high current pulsing conditions, it also provides long service life. Therefore, other than the power capability (V×I), the capacity or energy density of a particular active material needs to be considered. Due to these reasons, ε-SVO (Ag2V4O11) is a superior cathode active material for use in defibrillator cells due to its high volumetric capacity and acceptable power capability at all depths of discharge.
In U.S. Pat. No. 6,551,747 to Gan, the disclosure of which is incorporated herein by reference, a double screen sandwich cathode design for defibrillator applications is described. In this invention, the cathode electrode has two different cathode active materials and two current collectors. The first cathode active material is sandwiched between the current collectors. This assembly is then sandwiched between two layers of the second cathode active material. One example of a sandwich cathode electrode design is CFx active material positioned between two layers of current collector screen that, in turn, are sandwiched between two layers of SVO cathode material.
U.S. provisional application Ser. No. 60/204,477, filed May 16, 2000, the disclosure of which is incorporated herein by reference, describes cathode active materials for cells having sandwich cathode electrodes. A preferred cell design has a sandwiched SVO/CFx/SVO cathode. This cathode chemistry discharges in a very different manner in comparison to traditional Li/SVO defibrillator cells. In fact, the preferred SVO material for a traditional Li/SVO cell (ε-phase, Ag2V4O11) is not necessarily the most optimum SVO material for sandwich cathode electrode designs.
Accordingly, the present invention is directed to the use of a low volumetric capacity SVO material (γ-phase SVO, Ag1.6V4O10.8), instead of, or in addition to, the high volumetric capacity ε-phase SVO, in cells containing SVO/CFx/SVO sandwich cathode electrodes. This results in a defibrillator cell with higher power capability and longer service life than known by the prior art. For a further discussion of the preparation of γ-SVO and ε-SVO materials, reference is drawn to U.S. application Ser. No. 09/793,246, filed Feb. 26, 2001, the disclosure of which is incorporated herein by reference.